The present invention relates to a multilayered wiring board having a printed inductor and, more particularly, to a multilayered wiring board having a printed inductor which is suitable for improving frequency characteristics of a high frequency circuit of mobile radio terminal equipment or equipment for satellite broadcasting and for miniaturizing such equipment.
When forming a printed inductor on a multilayered wiring board conventionally, as shown in FIG. 4, a printed inductor 1 is formed on a grounding layer or electric power supply layer 2 with a dielectric layer 4 inserted between them. However, according to the structure shown in FIG. 4, particularly when the dielectric layer 4 is thin, the distance d between the printed inductor 1 and the grounding layer or electric power supply layer 2 is short, so that the stray capacity Cp existing between the printed inductor 1 and the grounding layer or electric power supply layer 2 increases. Assuming the inductance of the printed inductor 1 as L, the following relationship exists between the self-resonance frequency ft thereof and the stray capacity Cp: ##EQU1##
Therefore, there is a problem that as the stray capacity Cp increases, the self-resonance frequency ft of the printed inductor 1 decreases and the frequency characteristics deteriorates. In addition, when a printed inductor is formed on a wiring board with a dielectric layer inserted between them, the same problem is caused.
As shown in FIG. 4, a conventional multilayer wiring board having a printed inductor includes an electric power supply layer or grounding layer 3 and a layer 2. One layer 2 is the grounding layer, the layer 3 is an electric power supply layer. When the layer 2 is an electric power supply layer, the layer 3 is a grounding layer. A multilayered wiring board 7 includes the printed inductor 1, the electric power supply layer 2, an electric power supply or grounding layer 3, and a dielectric layer 4.
In Japanese Patent Laid Open Number 60-170911, a structure is proposed wherein a cavity is provided in the dielectric layer right under the printed inductor so as to increase the conductance of the printed inductor. In this construction, a conductor layer is provided on the opposite side of the dielectric layer to the printed inductor side. However, the part of the conductor layer right beneath the printed inductor is also removed. Thus, according to this construction, a cavity is formed in the dielectric layer by chemical etching or grinding, so that the manufacturing process is complicated and there is a possibility that the printed inductor is expensive to manufacture.
As mentioned above, there is a problem in the conventional multilayered wiring board having a printed inductor that the stray capacity between the printed inductor and the grounding layer or electric power supply layer (or wiring layer) increases, and the self-resonance frequency of the printed inductor decreases, and the frequency characteristics deteriorate. However, an inexpensive solution to the problems encountered in the prior art has not yet been proposed.